1. Field of the Invention
The present invention relates generally to the field of electronic devices and more particularly to over-voltage protection.
2. Description of the Prior Art
Semiconductor devices comprising a semiconductor die or chip are easily affected or destroyed by over-voltage events. Examples of over-voltage events include electrostatic discharge (ESD), line transients, and lightening strikes. Electrostatic discharge commonly occurs when a person carrying a static charge touches a semiconductor device. Line transients include power surges on AC power lines, and can also be caused by events such as closing a switch or starting a motor.
Voltage switchable materials, also known as nonlinear resistance materials, are materials that normally behave as dielectric materials, but upon application of a sufficient voltage, known as a switch voltage, will rapidly become electrically conductive. The ability of voltage switchable materials to switch between non-conductive and conductive states makes these materials well suited for over-voltage protection applications.
In the prior art, voltage switchable materials have been used for over-voltage protection in a number of different ways. For example, in Behling et al. (U.S. Pat. No. 6,570,765), microgaps are defined between contact portions and ground bars are filled with a voltage switchable material. Intrater (U.S. Pat. No. 6,433,394) teaches an integrated circuit chip with a plurality of conductive pads disposed around the periphery of a ground plane with a precision gap therebetween, and a protection device comprising a voltage switchable material placed over the integrated circuit chip. Shrier et al. (U.S. Pat. No. 6,542,065) teaches a variable voltage protection component comprising a reinforcing layer embedded with a voltage switchable material. In the prior art, therefore, achieving over-voltage protection through the use of voltage switchable materials requires redesigning semiconductor devices to include additional features, e.g., microgaps in Behling et al., a protection device in Intrater, and a reinforcing layer in Shrier et al.